I would like to add something to this.
On my Mobility 3650 (rv635/M86), I can normally boot with DPM without any issue
at all since 3.11.
However - sometimes (sporadically) I get the frosting screen when playing
Freecol.
When this happens, I can no longer boot with DPM enabled. If I don't disable
DPM in my bootloader, I always get the frosting screen when modesetting was
supposed to be happening.
In order to be able to boot again, I have to disable DPM, boot and leave the
computer on for a while (not sure how long, but less than 10 minutes is
insufficient and over 1 hour is enough).
If I do this, DPM will work again just fine.
I have no idea why this happens, and I am not sure what to log when it does
(actually, nothing gets logged on the failed boots).